A communication system has one principal function, to transmit information from a source to a destination. The information generated by the source is typically an electrical signal that changes with time.
The information signal is transmitted from the source to the destination over an appropriate medium, usually referred to as a channel. One method of altering the information signal to match the characteristics of the channel is referred to as modulation. The recovery of the information-bearing signal is called demodulation. The demodulation process converts the transmitted signal using the logical inverse of the modulation process. If the transmission channel were an ideal medium, the signal at the destination would be the same as at the source. However, the reality is that during the transmission process, the signal undergoes many transformations which induce distortion. A receiver at the destination must recover the original information by removing all other effects.
Most communications currently rely upon the conversion of an analog source into a digital domain for transmission and ultimately reconversion to analog form depending upon the type of information conveyed. The simplest digital representation is where the information in any bit time is a binary value, either a 1 or a 0. To extend the possible range of values that the information can be, a symbol is used to represent more than two possible values. Ternary and quaternary symbols take on three and four values respectively. The varying values are represented by integers, positive and negative, and are usually symmetric. The concept of a symbol allows a greater degree of information since the bit content of each symbol dictates a unique pulse shape. Depending upon the number of levels of a symbol, an equal number of unique pulse or wave shapes exist. The information at the source is converted into symbols which are modulated and transmitted through the channel for demodulation at the destination.
The normal processes of a communication system affect the information in a calculable and controllable manner. However, during the transmission from a source to a destination, a component that cannot be calculated is noise. The addition of noise in a digital transmission corrupts the signal and increases the probability of errors. Other signal corruptions that manifest themselves are multipath distortions due to natural terrain and manmade structures, and distances the signals travel which affect signal timing. The communication system needs to define the predictable transformations that the information signal encounters and during reception of the information the receiver must possess the means to analyze the predictable transformations that have occurred.
A simple binary transmission system could use a positive pulse for a logical 1 and a negative pulse for a logical 0, with rectangular pulse shapes transmitted by the source. The pulse shape received at the destination undergoes the aforementioned transformations including noise and other distortions.
To minimize the probability of error, the response of a filter used at the receiver is matched to the transmitter pulse shape. One optimal receiver, known as a matched filter, can easily determine whether a transmitted pulse shape is a logical 1 or 0 and is used extensively for digital communications. Each matched filter is matched to a particular pulse shape generated by the transmitter corresponding to a symbol. The matched filter is sampled at the symbol rate to produce an output that correlates the input pulse shape with the response of the filter. If the input is identical to the filter response, the output will produce a large value representing the total energy of the signal pulse. The output usually is a complex quantity that is relative to the input. The optimum performance of the matched filter depends on a precise replica of the received signal pulses which requires accurate phase synchronization. Phase synchronization can easily be maintained with the use of a phase-locked loop (PLL). Pulse synchronization, however, is a problem for matched filters. If the pulses are not time-aligned to one symbol time, intersymbol interference (ISI) appears.
An example prior art communication system is shown in FIG. 1. The system employs a technique known as code division multiplexing, or more commonly, as code-division multiple access or CDMA.
CDMA is a communication technique in which data is transmitted within a broadened band (spread spectrum) by modulating the data to be transmitted with a pseudo-noise signal. The data signal to be transmitted may have a bandwidth of only a few thousand Hertz distributed over a frequency band that may be several million Hertz. The communication channel may be used simultaneously by m independent subchannels. For each subchannel, all other subchannels appear as noise.
As shown, a single subchannel of a given bandwidth is mixed with a unique spreading code which repeats a predetermined pattern generated by a wide bandwidth, pseudo-noise (pn) sequence generator. These unique user spreading codes are typically orthogonal to one another such that the cross-correlation between the spreading codes is approximately zero. A data signal is modulated with the pn sequence to produce a digital spread spectrum signal. A carrier signal is then modulated with the digital spread spectrum signal, to establish a forward-link, and transmitted. A receiver demodulates the transmission and extracts the digital spread spectrum signal. The transmitted data is reproduced after correlation with the matching pn sequence. When the spreading codes are orthogonal to one another, the received signal can be correlated with a particular user signal related to the particular spreading code such that only the desired user signal related to the particular spreading code is enhanced while the other signals for all other users are not enhanced. The same process is repeated to establish a reverse-link.
If a coherent modulation technique such as phase shift keying (PSK) is used for a plurality of subscriber units, whether stationary or mobile, a global pilot is continuously transmitted by the base station for synchronizing with the subscriber units. The subscriber units synchronize with the base station at all times and use the pilot signal information to estimate channel phase and magnitude parameters.
For the reverse-link, a common pilot signal is not feasible. For initial acquisition by the base station to establish a reverse-link, a subscriber unit transmits a random access packet over a predetermined random access channel (RACH). The random access packet serves two functions. The first function is for initial acquisition when the subscriber unit is transmitting and the base station has to receive the transmission quickly and determine what is received. The RACH initiates the reverse-link to the base station. The second use of random access packets is for communicating lower data rate information rather than consuming a dedicated continuous voice communication channel. Small amounts of data such as credit card information are included in the data portion of the random access packet instead of call placing data. The information when sent to the base station can be forwarded to another communicating user. By using the random packet data portion to transport addressing and data, available air resources are not burdened and can be efficiently used for higher data rate communications.
A random access packet comprises a preamble portion and a data portion. The data may be transmitted in parallel with the preamble. In the prior art, the random access channel typically uses quadrature phase shift keying (QPSK) for the preamble and data.
The base station examines the received preamble for the unique spreading codes. Each symbol of the RACH preamble is spread with a pn sequence. Using matched filters, the base station searches continuously for those codes that correlate. The data portion contains instructions for a desired service. The base station demodulates the data portion to determine what type of service is requested such as a voice call, fax, etc. The base station then proceeds by allocating a specific communication channel for the subscriber unit to use for the reverse-link and identifying the spreading codes for that channel. Once the communication channel is assigned, the RACH is released for other subscriber units to use. Additional RACHs afford quicker base station acquisition by eliminating possible collisions between subscriber units simultaneously initiating calls.
Without a subscriber unit pilot signal providing pulse synchronization in the reverse-link, acquisition of the RACH from a mobile subscriber unit is difficult if a coherent coding technique such as PSK is used compounded with transmitting range ambiguity. Since a mobile subscriber unit is synchronized with the base station, the RACH preamble is transmitted at a predefined rate.
An example prior art preamble signature is defined by 16 symbols. A table of sixteen coherent RACH preamble signatures is shown in FIG. 2. Since each symbol is a complex quantity and has a pulse shape comprising 256 chips of the spreading pn sequence, each signature comprises 4096 chips. The complete RACH preamble signature is transmitted at a chipping rate of 4096 chips per millisecond or 0.244 chips per microsecond.
From the global pilot signal, each subscriber unit receives frame boundary information. Depending upon the distance from the base station to a subscriber unit, the frame boundary information suffers a forward-link transmission delay. A RACH preamble transmitted in the reverse direction suffers an identical transmission delay. Due to the propagation delay, the perceived arrival time of a RACH preamble at a base station is:
                                          Δ            ⁢                                                  ⁢            t                    =                                    2              ⁢                              (                distance                )                                      C                          ,                              where            ⁢                                                  ⁢            C                    =                      3.0            ×                          10              8                        ⁢                                                  ⁢            m            ⁢                          /                        ⁢                          s              .                                                          Equation        ⁢                                  ⁢        1            
Due to this inherent delay, the range ambiguity for a subscriber unit varies according to distance. At 100 m, the effect is negligible. At 30 km, the delay may approach the transmission time of 4 symbols. Table 1 illustrates the effect of round trip propagation delay.
TABLE 1Effect of range ambiguityround trip timerange (km)(msec)chip valuesymbol interval000150.0331371100.0672732150.1004102200.1335463250.1676833300.2008194
The first column is the distance in km between a mobile subscriber unit and a given base station. The second column is the round trip propagation delay of the RF signal in milliseconds from the base station to a subscriber unit and back. The third column shows the chip clocking position of the matched filter at the base station with time 0 referenced at the start of a transmitted frame boundary. The value represents when a first chip is received from a subscriber unit referencing the beginning of a frame boundary. The fourth column shows the expected location of the first output of the matched filter which occurs after assembling 256 received chips; (reference being made at the start of a frame boundary). A symbol may be output during any one of the first four symbol intervals depending on subscriber unit distance.
Since the base station is not synchronized with the subscriber unit and does not have a carrier reference, the base station does not know where in a received chip sequence the beginning of a RACH preamble symbol begins. The matched filter must correlate a total of 256 chips corresponding to a valid symbol pulse shape. As one skilled in this art knows, as the chips are received, the matched filter assembles 256 chips to produce a first output representative of the pulse shape. Consecutive outputs from the matched filter are generated for each subsequently received chip.
The mobile subscriber unit transmits the preamble part first to access the RACH from the base station. One from among sixteen signatures is randomly selected and one from among five time-offsets is randomly chosen to account for the range ambiguity during transmission. The mobile subscriber unit constantly receives a frame boundary broadcast from the base station. To request a RACH, the mobile subscriber unit transmits a random burst with an n×2 ms time-offset (where n=0, 1, . . . 4) relative to the received frame boundary as shown in FIG. 3. The time-offset (value of n) is chosen at random at each random access attempt.
Four received preamble signatures, a, b, c, and d are shown in FIGS. 4a-d received at the base station. Each signature arrives at one symbol duration (0.0625 ms) later due to round trip delay, with each signature representing a different distance between the base station and mobile subscriber unit. Only sixteen consecutive symbols have signal components, the other matched filter outputs represent noise. It is known that range ambiguity will destroy the orthogonality among signatures and degrade performance. The possibility exists that the base station receiver could confuse any combination from a possible nineteen outputs from the matched filter as an incorrect signature.
Accordingly, there exists a need for a CDMA transmission and detection scheme that is accurate notwithstanding communication distance and the effects of Doppler.